This invention relates to underwater seismic exploration methods.
Seismic exploration contemplates the collection of data which will characterize or define subsurface geological formations. For land seismic exploration, such data is commonly obtained by producing a seismic disturbance (e.g., detonating an energy charge) at or below the surface of the earth and then detecting and recording the resultant seismic waves which are reflected from the reflecting interfaces of the subsurface formations. An array of seismometers spaced at varying distances from the disturbance, is used to detect and record the seismic waves.
The seismic record--termed a seismogram--consists of a plurality of traces, each of which is generated by a different seismometer. Each trace is a time varying representation of the seismic waves received by the corresponding seismometer.
Although land and marine (hereinafter referred to as "underwater") seismic exploration involve many of the same techniques such as those described above, underwater exploration presents peculiar problems not generally found in land exploration. In underwater exploration, the seismic disturbance is generally produced below the surface of the water and the resulting seismic reflections are detected by a string of seismic detectors (such as hydrophones) towed underwater by a survey boat. The seismic detectors typically are submerged in a line behind the boat at a depth equal to about one fourth the wave length of a seismic wave.
One problem with underwater seismic exploration is the generation of certain undesirable seismic wave reflections which cause spurious and unwanted signals to be recorded on the seismogram. Two of such undesirable wave reflections are the water surface reflection (from the water/air interface), and the water floor reflection (from the water/land interface). These reflections may be of such magnitude and may arrive at such time as to obscure the desirable seismic waves reflected from the reflecting interfaces of the formations below the water floor.